U.S. Pat. No. 6,756,139 described a coated conductor architecture based on ion beam assisted deposited magnesium oxide (IBAD-MgO), which comprises four or five separately deposited layers between the metal substrate and the superconducting layer. Among the described structures are included: (1) a structure including a substrate/a layer of aluminum oxide (Al2O3)/a layer of yttrium oxide (Y2O3)/a layer of IBAD-MgO/a homoepitaxial layer of MgO/a layer of strontium ruthenate (SrRuO3)/a layer of yttrium barium copper oxide (YBCO); and (2) a structure including a substrate/a layer of erbium oxide (Er2O3)/a layer of IBAD-MgO/a homoepitaxial layer of MgO/a layer of SrRuO3/a layer of YBCO. Other architectures have employed strontium titanate in place of strontium ruthante or mixtures of strontium titanate and strontium ruthante.
Although excellent critical current density values have been achieved using either of these architectures, the homoepitaxial MgO layer has some drawbacks. The deposition rate of the homoepitaxial layer is undesirable slow, especially with a deposition process such as RF sputter deposition. Where the deposition process for the homoepitaxial MgO is reactive sputter deposition it can be difficult to precisely control the film thickness and the stoichiometry. Also, while much benchscale research on such materials has generally employed pulsed laser deposition (PLD) with production of high quality stoichiometric films, PLD is not presently preferred for commercial production of coated conductors.
Due to these problems, a substitute material for the homoepitaxial MgO layer is desired. Additionally, it is optionally desired that such a replacement material can be deposited using sputter deposition of a metal target.
After careful experimentation by the present inventors, a number of substitute materials have now been developed for the homoepitaxial MgO layer, including some substitute materials that can be deposited by high rate reactive sputter deposition.